1. Field of the Invention
This invention relates to methods and apparatus of creating propulsion; in particular this invention relates to thrust producing apparatus; and most particularly, this invention relates to rocket engines.
2. State of the Art
Rocket engines of one sort or another have existed since the Chinese invented them some time in the twelfth century AD. These earliest rocket engines were solid fueled, that is they burned some solid propellant, for example, packed black powder, fast enough to provide enough thrust to lift the rocket off the ground and propel it into the air. The charge of fuel in these rockets was a solidly packed hollow cylinder of propellant, and the combustion gases exited through the hollow formed by the packed fuel at high velocity. Although these first rockets were little more than toys, the same fundamental principles that applied to these rockets apply to all rockets.
Another kind of rocket engine was invented earlier this century when Robert Goddard, among others, experimented with and perfected one kind of liquid fueled rocket engine--a kind which will be termed the bell nozzle engine herein. This term will cover all conical or bell shaped housings, whether or not they conform exactly to a bell nozzle geometry in detail or not. The Germans used the rockets propelled by the bell nozzle engine as a weapon of war--the V-2--during the Second World War to target London from across the English Channel. The engine used in the V-2 was entirely typical of liquid fueled rocket engines then and now. It included a hollow conical or bell shaped member, open at the large end, and having, at the narrow, closed end of the bell, a fuel and oxidizer entrance or fuel and oxidizer injectors. The fuel was burned with the oxidizer near the fuel and oxidizer entrance within the conical chamber--for example, in the case of the V-2, liquid ethanol was combusted with liquid oxygen(LOX)--and the hot, high pressure combustion gases formed exited the large open end of the cone to provide thrust that lifted the rocket propelled vehicle from the ground.
As noted, in general, these engines comprised hollow conical combustion chambers and nozzles disposed below fuel and oxidizer injectors. The injection apparatus into the conical combustion chamber can be quite simple; indeed, often a plate affixed to the narrow end of the conical chamber and interpenetrated with a plurality of apertures for the fuel and oxidizer to pass through suffices. In such simple rocket engines, the fuel is basically poured into the rocket combustion chamber through the apertures.
On the one hand, these bell nozzle style rocket engines have accomplished much; they have, for example, gotten humans to the moon and back. But, on the other hand, the engines used in the most modern rocket propelled vehicles--for example, the space shuttle--are little more than highly refined versions of the same old bell nozzle engine design that has been used for decades. And, as might be expected, this old, albeit simple and reliable, rocket engine design suffers from some inherent design limitations. Among them are, first, the velocity of the gases exiting the conical engine are influenced by the ambient pressure of the atmosphere they exit into. This requires one nozzle design for efficient high attitude and vacuum use and different design for the most effective low altitude use, that is primarily the time of lift off. This effectively precludes the use of a single design of bell nozzle style engine for a single stage that both takes off at near sea level and then acquires earth orbit. Second, the velocity of the gases exiting the nozzle, relative to the nozzle, is essentially a function of the Boltzmann average velocity distribution for molecules of the exhaust gas at the combustion temperature. The only effective method to increase the exit velocity of the gases in a bell nozzle style engine is to increase the temperature of combustion--and that temperature is, of course, dictated by the chemistry of the gases chosen for combustion.
A different kind of liquid fueled rocket engine has been developed over the last twenty five years, which will be termed the plug-nozzle engine hereinafter. This engine design evolved from experimental and analytical studies and the testing of various rocket nozzle configurations, combustion designs and improved, simplified engine cycles. The most obvious visual difference between the conventional bell and aerodynamic spike nozzle is that the bell design controls the primary flow of exhaust gases by the inner surface of a contoured restraining chamber wall, while in the aero-spike the primary flow is controlled by the atmospheric pressure acting in concert with an outer wall of the aero-spike to produce the thrust force. In many aero-spike engines, a secondary flow is introduced through the center of the annulus of the aerodynamic nozzle, increasing the base pressure and adding to the efficiency of the nozzle. A short, high performance rocket nozzle results.
The plug nozzle engine, combining the advanced nozzle and combustion chamber design, can result in a high-performance engine as much as 75 percent shorter than an equivalent bell nozzle engine. The advantages of such an engine include shorter, lighter inter-stage structures, plus a viable design choice between a shorter overall vehicle or significantly longer, larger capacity propellant tanks.
But plug nozzle engines suffer from some limitations. First, the flow out of a circular plug nozzle engine is largely symmetrical and not easily redirected. Linear plug nozzle engines, essentially circular engines that have been extended greatly along one axis, can be somewhat controlled relative to one dimension (as usually portrayed, a linear plug nozzle engine is oriented to enable adjustment of the pitch of the vehicle), but the linear plug nozzle propelled vehicles still require separate thrusters to enable yaw.
Another problem with conventional plug nozzles is that they create a region of subsonic flow just aft of the nozzle. As mentioned above, attempts to rectify this problem have been made that involve venting combusted gases from the center of the nozzle. Still this problem results in the formation of troublesome dynamic shock waves trailing the nozzle.
Therefore, it can be seen that the conventional bell and plug nozzles both present significant problems to those trying to develop the single stage to orbit concept.